Minor fix in Ganesh shader generation.

src/gpu/gl/GrGLSL.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrGLSL.h"
 #include "GrGLShaderVar.h"
 #include "SkString.h"
@@ skipping 68 matching lines @@
                       const char* vec4VarName,
                       const GrGLSLExpr4& mulFactor) {
     if (mulFactor.isOnes()) {
         *outAppend = SkString();
     }
 
     append_tabs(outAppend, tabCnt);
 
     if (mulFactor.isZeros()) {
         outAppend->appendf("%s = vec4(0);\n", vec4VarName);
+    } else {
+        outAppend->appendf("%s *= %s;\n", vec4VarName, mulFactor.c_str());
     }
-    outAppend->appendf("%s *= %s;\n", vec4VarName, mulFactor.c_str());
 }